The present invention generally relates to the field of power amplifiers used in wireless systems. More specifically, it relates to a protection circuit for wireless handset amplifier circuits.
A wireless system requires an air interface or antenna to transmit radio wave energy. Wireless systems such as mobile systems, which encompass systems based on Global System for Mobile Telecommunication (GSM)/EDGE standards, and the like, have to handle high power. The high power requirements of mobile systems illustrate the need for conversion of a low-power radio frequency signal to a high power signal. For this purpose, a Power Amplifier (PA) is used for driving the antenna in a mobile system. Ideally, a PA should be completely matched to the antenna so that maximum amount of energy is transmitted. The matching of an antenna and the corresponding PA is expressed in terms of a Voltage Standing Wave Ratio (VSWR), which is the ratio of maximum to minimum voltages in a standing wave. However, the impedance of antenna can vary due to environmental factors or antenna damage. This causes a mismatch between the antenna and the PA, as a result standing waves are formed by forward and reflected waves to and from the antenna giving rise to constructive or destructive interference. Ideally, a VSWR value of 1:1 indicates complete match between PA and antenna for maximum power transmission from antenna. High values of VSWR indicate a load mismatch, and load mismatch can cause damage to the power amplifier due to voltage breakdown. Additionally, other factors, such as power supply over voltage, temperature and excessive input drive demand the PA to be highly rugged. Thus, it is desirable to have a PA that is rugged and can withstand high VSWR load mismatch conditions. One possible solution to increase the ruggedness of power amplifiers is to design the PA to have a high output voltage breakdown capability. However, increase in the breakdown capability of the PA comes at the expense of performance factors such as Power Added Efficiency (PAE). Thus, a better solution is desired.
It is well known in the art to have a matching component or a matching network between a PA and antenna. One solution is to use an isolator, which transmits radio frequency power in one direction only. Thus, it can protect the PA from the effects of a mismatched load. However, an isolator adds some series loss, increases cost and grows the solutions footprint. A matching circuit cannot prevent voltage peaks at the output of a PA from causing damage to the PA due to voltage breakdown. Thus, it is desired to have a protection circuit, which can prevent damage to the PA due to voltage breakdown.